With the continued growth of volume X-ray imaging with CBCT systems, there are increasing demands to estimate the spectra emitted from a CBCT scanner. For example, it is desirable for image quality assurance and quantitative imaging purposes to periodically monitor and characterize the output spectra and to make sure the X-ray tube output is consistent and accurate. The estimated spectra can also be used as prior information in poly-energetic reconstruction (or spectral reconstruction) and with scattered radiation correction to improve image quality. In addition, in patient dose and risk estimation, spectra are indispensable information for Monte Carlo based simulation methods.
Spectral calibration of X-ray output using conventional methods is costly and time-consuming, requiring special equipment and procedures that are not available at every installation. There are problems that make the spectral estimation of the CBCT a challenge.
Accurate measurements can be difficult to obtain. This is in part because of the large opening of the X-ray beam with CBCT. Scattered radiation may contaminate the detector signals and make calibration results inaccurate.
Conventional methods for spectral calibration of the X-ray equipment can be labor-intensive and tedious. In conventional practice, the cone-beam X-ray is first collimated into pencil-beam shape to reduce scatter, and then a series of projections are acquired using different thickness of metals (e.g., Al or Cu) or step phantoms. A spectrometer can be used to measure the spectrum with one exposure; however, the alignment task is complex. It proves to be very difficult to align the pencil-beam collimator of the CBCT scanner with the collimator of the spectrometer. There are special setup considerations because of the high sensitivity of spectrometer instrumentation. The spectrometer is costly, and not all imaging centers or clinics can afford such instrumentation or the specialized staff needed for setting up and operating this equipment.
To address the above problems, this disclosure proposes an efficient spectral calibration technique that operates by imaging a phantom and iteratively approximating the inherent filtration of the system until a close estimate of the spectral content of the X-ray energy is obtained.